Group III nitride semiconductor crystals, such as aluminum nitride, gallium nitride and indium nitride, have a wide range of band gap energy value. Their band gap energies are around 6.2 eV, around 3.4 eV, and around 0.7 eV, respectively. Mixed crystal semiconductor of any composition may be formed of these group III nitride semiconductors, and according to the composition of mixed crystal, band gaps having values between the above band gaps can be obtained.
Therefore, by using group III nitride semiconductor crystals, it is possible to make light emitting devices having a wide range of emitting right from infrared light to ultraviolet light in principle. Specifically, development of light emitting devices having aluminum-based group III nitride semiconductors (mainly aluminum gallium nitride mixed crystal) has been vigorously promoted recently. The use of an aluminum-based group III nitride semiconductor enables emission of short wavelength light in the ultraviolet range, which makes it possible to manufacture light emitting light sources such as ultraviolet light emitting diodes for white light sources, ultraviolet light emitting diodes for sterilization, laser for read/write of high density optical disc memories, and laser for communication.
A light emitting device having a group III nitride semiconductor (e.g. aluminum-based group III nitride semiconductor) may be formed by sequentially stacking thin films of semiconductor single crystal (specifically, thin films to be n-type semiconductor layer, light emitting layer, and p-type semiconductor layer) of around several microns in thickness onto a substrate, in the same way as in forming conventional semiconductor light emitting devices. The thin films of such a semiconductor single crystal may be formed by crystal growth methods such as Molecular Beam Epitaxy (MBE), and Metalorganic Chemical Vapor Deposition (MOCVD). As to a group III nitride semiconductor light emitting device as well, such methods are tried to be applied to form a preferable layer structure as a light emitting device.
At the moment, in manufacturing a group III nitride semiconductor light emitting device, generally employed is sapphire substrate, in view of crystal quality as a substrate, permeability of ultraviolet light, mass productivity and cost. However, when a group III nitride is grown on a sapphire substrate, crystal defects (misfit dislocation), cracks and the like occur due to differences in lattice constant, thermal expansion coefficient and the like between the sapphire substrate and the group III nitride to form semiconductor stacked layer (e.g. aluminum gallium nitride), which results in a degradation of light emitting performance of the device.
In order to solve these problems, in forming a semiconductor stacked film, it is desirable to use a substrate having a lattice constant and thermal expansion coefficient close to that of the semiconductor stacked film. As a substrate to form a group III nitride semiconductor thin film, group III nitride single crystal substrate is most suitable. For example, as a substrate to form an aluminum-based group III nitride semiconductor thin film, aluminum nitride single crystal substrate and aluminum gallium nitride single crystal substrate are most preferable.
In order to use a group III nitride single crystal as a substrate, it is preferable that the single crystal has a certain degree of thickness (e.g. no less than 10 μm), in view of mechanical intensity. MOCVD is suitable for manufacturing a group III nitride single crystal substrate, because crystals grow faster by MOCVD compared to MBE. As a growing method of a group III nitride single crystal of higher growth rate of film formation than MOCVD, Hydride Vapor Phase Epitaxy (HVPE) is known (see Patent Literatures 1 to 3). HVPE is not suitable for precise control of film thickness compared to MBE and MOCVD, whereas HVPE can grow single crystals of good crystallinity at a high growth rate of film formation. Thus, HVPE is especially suitable for mass production of single crystal substrates. The growing of a group III nitride single crystal by MOCVD and HVPE is carried out by supplying a group III source gas and a nitrogen source gas in a reaction vessel, and reacting the gases on a heated substrate.
Relating to manufacturing of group III nitride single crystals, for example Patent Literature 4 discloses a hydride vapor phase epitaxy apparatus including a reaction vessel, a group III source gas generation part to generate a group III halide gas, and a group III halide gas introducing pipe to supply the group III halide gas to a reaction zone of the reaction vessel. Patent Literature 4 describes that an inlet at an end of the group III halide gas introducing pipe penetrates an end wall of an outer chamber of the reaction vessel, and the inlet at the end of the group III halide gas introducing pipe joins to a first nozzle arranged inside an inner chamber of the reaction vessel.